Experiments on Fatigue Strength and Fatigue Failures in Spot-Welded Carbon Steel

Abstract

We performed the fatigue tests of single spot-welded steel SS41P under completely reversed direct stress at room temperature. Two modes of failures, that is, the through-the-thickness failure and shear failure at the interface of sheets were observed through these tests. The former occupied 84% of total failures and had higher frequency of occurrence at low stress level than at high stress level. We postulate that through-the-thickness failure will occur when a maximum principal stress reaches the limit value which depends on a number of stress cycles. In order to estimate this stress, the following stress components were taken into consideration, the shearing stress at the interface of sheets and normal stress, which are in the direction of loading, and the normal stress in the thickness direction. In addition to the original residual stress, the stress in the thickness direction includes the stresses which are developed by bending due to the attachment of the specimen on testing machine and the eccentric loading of external force during the fatigue test. In a S-N diagram, in which the nominal stress is adopted as ordinate, a considerable scattering of plotted data is shown and it is difficult to recognize the effects of such factors as diameter of electrode and spot-welding conditions. In a modified S-N diagram, however, in which the maximum principal stress is taken as ordinate, its scattering of data is much reduced. In this diagram the S-N curve appears to depend primarily on a diameter of faying area rather than on spot-welding conditions. In comparison with the S-N curve of the mother material, we obtained the strength reduction factor β=2.73 for the number of stress cycles until the through-the-thickness failure Nf=5×106 is reached, and we consider that the main cause of the low fatigue strength in the spot-welded joint may be the high stress concentration at the origin of the crack.